Selective Electrochemical Production of Formate from Carbon Dioxide with Bismuth-Based Catalysts in an Aqueous Electrolyte

For the efficient electroconversion of CO₂ to formate, CO and H₂ evolution must be suppressed. Herein, carbon-supported BiO_x nanoparticles (BiO_x/C) were investigated as a potential candidate for CO₂ reduction. In bicarbonate solutions, the BiO_x/C catalysts exhibited a high Faradaic efficiency of 93.4% for formate from -1.37 to -1.70 V versus Ag/AgCl with a negligible amount of CO and H₂. Stable partial current densities and high Faradaic efficiencies were also achieved in 0.5 M NaCl (12.5 mA cm^-2 and 96.0%, respectively). The possible reaction pathways and kinetic parameters of formate formation were examined using systematic electrochemical methods, including Tafel, pH dependence, and in situ X-ray absorption near-edge structure analyses. From the results of these mechanistic studies, we propose that dual mechanisms are functional on the BiO_x/C catalysts. Specifically, a two-electron and one-proton transfer reaction to adsorbed CO₂ or a chemical proton transfer reaction to CO₂^- anion are the possible rate-determining steps (RDSs) at low potentials, whereas a one-electron transfer reaction to CO₂ is the RDS at high potentials.